more helpful "PCH file built from a different branch" message

Summary:

The PCH file built from a different branch error does not provide a hint on how to fix it. Add such a hint.

Description:

When I try to build with the wrong precompiled header (PCH), I get this:

~/github/stan-dev/cmdstan (develop)$ make ~/temp2/simple

--- Translating Stan model to C++ code ---
bin/stanc  --o=/Users/bcarpenter/temp2/simple.hpp /Users/bcarpenter/temp2/simple.stan

--- Compiling, linking C++ code ---
clang++ -std=c++1y -Wno-unknown-warning-option -Wno-tautological-compare -Wno-sign-compare -D_REENTRANT -Wno-ignored-attributes      -I stan/lib/stan_math/lib/tbb_2020.3/include    -O3 -I src -I stan/src -I lib/rapidjson_1.1.0/ -I lib/CLI11-1.9.1/ -I stan/lib/stan_math/ -I stan/lib/stan_math/lib/eigen_3.3.9 -I stan/lib/stan_math/lib/boost_1.78.0 -I stan/lib/stan_math/lib/sundials_6.1.1/include -I stan/lib/stan_math/lib/sundials_6.1.1/src/sundials    -DBOOST_DISABLE_ASSERTS          -c -include-pch stan/src/stan/model/model_header.hpp.gch -x c++ -o /Users/bcarpenter/temp2/simple.o /Users/bcarpenter/temp2/simple.hpp
error: PCH file built from a different branch ((clang-1316.0.21.2.5)) than the compiler ((clang-1400.0.29.102))
1 error generated.
make: *** [/Users/bcarpenter/temp2/simple] Error 1

This is great that it says what's wrong, but it would be even better if it suggested how to fix the problem. I'm pretty sure I need to just delete a .pch file, but I don't know where it's at. A recursive search didn't work:

~/github/stan-dev/cmdstan (develop)$ ls -R -l | grep pch
-rw-r--r--   1 bcarpenter  1595  3773 Aug 10 12:07 pchip.hpp
-rw-r--r--  1 bcarpenter  1595   340 Aug 10 12:07 pch.hpp
-rwxr-xr-x  1 bcarpenter  1595  1175 Aug 10 12:07 cppcheck.sh
-rw-r--r--   1 bcarpenter  1595    537 Aug 10 12:07 pch.hpp
-rw-r--r--   1 bcarpenter  1595   2717 Aug 10 12:07 pch.jam
-rw-r--r--   1 bcarpenter  1595   2848 Aug 10 12:07 pch.py
~/github/stan-dev/cmdstan (develop)$ 

Reproducible Steps:

See above, but to reproduce it, you need to have two compilers, one of which built the pch file and a different one to tickle this error.

Current Output:

See above.

Expected Output:

Some hint as to where the pch file is that I need to delete. Even better, just automatically delete and rebuild after detecting this error.

Additional Information:

There are a lot of pages where people brought up this problem on our forums or in issues. The advice in the first hit on the forums didn't work, which suggested just running make clean. Here's what I get:

~/github/stan-dev/cmdstan (develop)$ make clean
rm -f -r test
rm -f 
rm -f 
rm -f 
rm -f 
~/github/stan-dev/cmdstan (develop)$ make ~/temp2/simple

--- Compiling, linking C++ code ---
clang++ -std=c++1y -Wno-unknown-warning-option -Wno-tautological-compare -Wno-sign-compare -D_REENTRANT -Wno-ignored-attributes      -I stan/lib/stan_math/lib/tbb_2020.3/include    -O3 -I src -I stan/src -I lib/rapidjson_1.1.0/ -I lib/CLI11-1.9.1/ -I stan/lib/stan_math/ -I stan/lib/stan_math/lib/eigen_3.3.9 -I stan/lib/stan_math/lib/boost_1.78.0 -I stan/lib/stan_math/lib/sundials_6.1.1/include -I stan/lib/stan_math/lib/sundials_6.1.1/src/sundials    -DBOOST_DISABLE_ASSERTS          -c -include-pch stan/src/stan/model/model_header.hpp.gch -x c++ -o /Users/bcarpenter/temp2/simple.o /Users/bcarpenter/temp2/simple.hpp
error: PCH file built from a different branch ((clang-1316.0.21.2.5)) than the compiler ((clang-1400.0.29.102))
1 error generated.
make: *** [/Users/bcarpenter/temp2/simple] Error 1

Current Version:

v2.30.1

Start with the make invocation and the compile command shown in the report, especially the -include-pch argument referencing stan/src/stan/model/model_header.hpp.gch. Reproduce the compiler-mismatch failure, then trace where this error is surfaced. Done means the output identifies the PCH location or automatically rebuilds it, with the failure behavior covered by a test.